The invention relates to a foldable wheelchair having side frames arranged on both sides of the wheelchair, a cross-strut assembly and seat tubes for receiving a seat cover, wherein the cross-strut assembly is mounted, in regions remote from the seat tube, about parallel pivot axes in the side frame, and the axis of rotation of the struts of the cross-strut assembly is arranged parallel to these pivot axes of the cross-strut assembly, and the cross-strut assembly is connected to the seat tubes, which, in the unfolded position of the wheelchair, are received by seat tube bearings of the side frames, wherein the wheelchair has guide levers on both sides, and the respective guide lever is mounted, in a first region, in a strut of the cross-strut assembly, in a region of the cross-strut assembly between the axis of rotation of the struts of the cross-strut assembly and the seat tube, and this guide lever is mounted at a distance therefrom, in a second region, in the side frame assigned to this seat tube so as to be pivotable about a pivot axis, wherein this pivot axis is arranged parallel to the pivot axes of the cross-strut assembly in the side frames.
Similar foldable wheelchairs are known from the prior art.
Basically, the folding mechanism of modern folding wheelchairs from virtually all wheelchair manufacturers is realized substantially by means of a so-called cross-strut assembly. These are two articulatedly interconnected struts which form an axis of rotation, wherein the articulation point is situated approximately in the center of the struts. Also known are so-called double cross-strut assemblies, in the case of which at least one strut part is composed of two struts arranged in parallel.
However, only the single design will be considered below.
Situated at the upper end of the struts are the seat tubes. They form a right angle with the struts, are oriented in a direction of travel and serve for the fastening of the seat cover. In the unfolded state, that is to say the usage situation of the wheelchair, said seat tubes are fixed in seat tube bearings fastened to the side frames.
At the lower end of the struts, these are articulatedly connected to the lower region of the side frames, wherein the axes of rotation are oriented in a direction of travel. Owing to the criss-crossing articulated connection of the struts, said struts are, in the unfolded state of the wheelchair, in each case connected at the lower end to one side frame and mounted at the upper end in the other side frame.
During the folding-together of the wheelchair, the seat tubes are, owing to tension on a seat cover of the wheelchair, guided upwardly out of the seat tube bearings, wherein the cross-strut assemblies and thus the wheelchair as a whole are folded together.
Since, after the release of the seat tubes from their bearings, the side frames are articulatedly connected only in the lower region to the cross-strut assembly, a guide lever is required in each case in order to prevent the side frames from tilting away to the side in uncontrolled manner and being guided in a controlled manner. Said guide levers are in each case articulatedly connected at one side to the upper region of a side frame and at the other side to that strut of the cross-strut assembly which is connected to the oppositely situated side frame. The axes of rotation of the guide lever are likewise oriented parallel to the direction of travel. In particular, the articulation point in the struts of the cross-strut assembly are selected such that the side frames are parallel to one another both in the unfolded state and in the folded-together state.
This is however not the case at the start of the folding process: owing to the geometric conditions arising from the design, the connecting line of the axes of rotation of a guide lever in the unfolded state of the wheelchair (usage situation) is, as viewed from the axis of rotation on the side frame, directed downward—below the horizontal in relation to the normal orientation of the wheelchair. During the folding process, the guide levers are rotated into the horizontal as a result of the movement of the cross-strut assembly, whereby the respective horizontal spacing from the axis of rotation at the cross-strut assembly to the axis of rotation at the side frame is increased, and the upper regions of the side frames are pushed outward, away from the cross-strut assembly. Since the side frames are articulatedly connected in their lower region to the cross-strut assembly, they perform a corresponding rotational movement there, such that the sagittal planes of the side frames moved toward one another at an angle from the parallel, and assume a V-shaped position. Thus, in this early phase of the folding process, the back tubes of the wheelchair move away from one another and generate a high tension on the back cover of the wheelchair if the latter does not have a correspondingly large degree of slack in the unfolded state of the wheelchair. This effect is intensified, in accordance with the V shape, with the height of the back cover.
In practice, this effect is counteracted in two ways, or by means of the combination thereof:                1. The back cover is provided with a degree of slack in the unfolded state of the wheelchair, which permits the V-shaped movement of the back tubes away from one another.        2. The guide levers, or the attachment thereof to the cross-strut assembly, are structurally designed such that they can bend during the folding process and thus compensate the approximately parallel guidance of the side frames that is forced by a taut back cover.        3. The articulated connection of the cross-strut assembly is produced with play, such that the cross-strut assembly itself can, together with the side frames connected thereto, be moved into a forwardly directed V shape, that is to say a V shape directed in the direction of travel, and thus compensate the movement of the back tubes away from one another.        
The latter possibility is not applicable to modern, energy-efficient wheelchairs, because such a wheelchair must be as rigid as possible and must have the least possible play in its articulated connections.
The first two possibilities, or the combination thereof, are however associated with a considerable mechanical load on the guide lever and/or on the flexible attachment elements thereof on the cross-strut assembly and the cross-strut assembly articulation point. Furthermore, these methods of the combination thereof can only partially prevent the state effect. Complete prevention would require an unachievable deformation of guide levers and/or of the flexible attachment elements thereof on the cross-strut assembly.